1. Field of the Invention
The invention relates to a back light unit, and more particularly, to a back light unit with high thermal dissipation.
2. Description of the Prior Art
A back light unit is one of the key elements of liquid crystal display (LCD) panel and is applied to many modern electric devices, such as a digital camera, mobile phone, person digital assistant (PDA), computer monitor, and flat television. In general, the back light unit is set in the backside of a display panel. The back light unit comprises a light source generator, a diffuser positioned on the light source generator for uniformly scattering the light beams from the light source generator, and a plurality of prisms positioned on the diffuser to raise the brightness and make the lightness of the whole display panel uniform. There are two types of back light units: direct-underlying type and edge light type. In a direct-underlying type, the light source generator is positioned in the backside of the display panel. Because there are more spaces in the back-side of the display panel, the light source generator may contain more than two florescent tubes to increase the lightness. As a result, a display panel requiring higher lightness or larger size, such as LCD monitor or LCD television used outside, adopts the direct-underlying type being its back light unit. On the other hand, an edge type back light unit comprises a light resource set in a flank of the display panel so as to reduce the thickness of the display panel. Therefore an edge type back light unit can satisfy the requirements of being thin, light, and saving power when applied to an LCD monitor or notebook.
FIG. 1 is a section view of a direct-underlying type back light unit 10 according to the prior art. The direct-underlying type back light unit 10, which is set in a back-side of a display panel 20, comprises a light source generator 12, a diffuser 16 set between the light source generator 12 and the display panel 20, a housing 18 enclosing the light source generator 12, and a reflecting sheet 14 set under the light source generator 12 and fixed on the inside surface of the housing 18. The light source generator 12 is used to supply a light source to emit light beams to the display panel 20. The reflecting sheet 14 is used for reflecting light beams from the light source generator 12 up to the diffuser 16 to raise the utility of the light beams. The diffuser 16 can further scatter the light beams to provide uniformly scattered light beams to the display panel 20. The housing 18 is used to position the diffuser 16, reflecting sheet 14, and the light source generator 12. The direct-underlying type back light unit 10 according to the prior art usually further comprises a diffusion film and a plurality of prisms (not shown) set on the diffuser 16 to correct the lightness and uniformity of the back light unit, wherein the amount and position of the diffusion film and the prism is different according to the design and requirements of the display panel.
For achieving the goal of high lightness and thinness, the light source generator of a back light unit is usually set in an airtight chamber, thus it is hard to radiate the heat produced by the display panel and the back light unit to the external environment when the display panel is operated. As the operation time becomes longer, the heat in the back light unit is accumulated and it results in the temperature around the fluorescent tubes being too high to maintain normal operation, so that the performance of the display panel will be reduced (for example, a portion of the image may flash) and the lifetime of the elements of the back light unit will be shortened.
For improving the heat dissipation of a back light unit, JPN 2001-297623 disclosed using vent holes and a fan to be the heat transferring device of the back light unit. It taught setting a plurality of small holes on the housing of the back light unit to serve as vent holes, and to set a fan outside the vent holes to create a heat convection between the back light unit and the external environment. However, as the vent holes and the fan are set, the noise of the van occurs and the dust easily intrudes into the back light unit to reduce the utility of the light. In JPN 2001-216870, a radiating area of the back light unit is increased to improve the heat dissipation of the back light unit, which means the housing of the fluorescent tubes electrodes is made as bended shapes. However, there is still a limitation of increasing the radiating area according to JPN 2001-216870, thus only a small amount of heat can be dissipated, and the heat accumulation problem of the back light unit is not solved yet. Therefore how to improve the heat dissipation of the back light unit and simultaneously raise the utility of light of the display panel, and furthermore to improve the display performance and extend the lifetime of the device is an important issue for manufactures of back light units.